As is known, the differential of a motor vehicle is composed of a set of gearwheels which are connected together, two of which being integral with the two half-shafts on which the drive wheels are keyed and others of which (usually two or four), called side gears, being keyed onto a cross journal having its ends constrained to a box which is mechanically connected to the engine of the motor vehicle, which causes it to rotate about the longitudinal axes or rather the common longitudinal axis of the said half-shafts.
It is considered unnecessary to explain in detail the operation of a differential which is well-known to persons skilled in the art. In a few words, it allows the drive wheels to rotate at different speeds when a motor vehicle is travelling along a curved trajectory, regulating the driving torque acting on each half-shaft.
As is known, the operational nature of a differential results in the drawback that the entire driving torque is transferred to the wheel of the two drive wheels which, not gripping on a slippery or frozen surface, “slips” without causing the vehicle to move forwards. The other wheel, which, having a better grip on the ground, could move the vehicle forwards, is imparted a substantially zero torque and this series of circumstances has the effect that, when one of the drive wheels gets stuck, it is no longer possible to move the vehicle.
In order to avoid this known drawback, numerous devices which perform so-called locking of the differential have been devised: U.S. Pat. No. 5,171,192 (Schlosser et al.), U.S. Pat. No. 5,947,859 (McNamara) and U.S. Pat. No. 5,591,098 (Jones et al.) may be regarded as the most relevant examples of the state of the art.
A common feature in nearly all these documents is the fact that, in order to achieve locking of the differential, the procedure adopted involves modifications inside the differential housing, with constructional complications and relatively high costs.